1. Field of the Invention
The invention relates to a new type of static echographY probe and the process for manufacturing such a probe. The invention also relates to an echography apparatus incorporating such a probe.
2. Description of the Prior Art
The most widely used echography probes at the present time are sectorial sweep probes, that is to say comprising either an oscillating mobile assembly or several transducers mounted on a wheel and switched as they travel past an emission window. The qualities of these probes are their speed of acquisition and their fundamental simplicity which results in relatively simple and inexpensive signal processing means. The coupling surface is relatively small, so that the probe may be disposed between two ribs of the patient for cardiac observation. On the other hand, the life span of these probes is limited.
Systems using liner arrays of transducer elements are essentially reserved for observing adominal regions, because of the large dimensions of the probe. In these systems, the elements (or groups of elements) are successively switched so as to provide a sweep perpendicular to the row of elements. The technology of linear array probes has been used for observations of the thoracic cage, by reducing the coupling surface of the probe and distributing delays (on emission as at reception) between the transducer elements of the array so as to reconstitute a sectorial sweep, i.e. so as to emit and receive in convergent directions inscribed in a sweep range. This technology, known under the name of phased array, provides a static probe whose coupling surface has sides of no more than 20 mm. However, the processing electronic equipment is very expensive. In fact, the delays to be provided (by delay lines, on the reception side at least) may reach 10 microseconds and an acceptable control of the directivity is only possible if these delays are provided with a tolerance of 10 nanoseconds. Now, at the present time, such an accuracy is obtained only for delays of two to three microseconds at most. To overcome this problem, a frequency change may be operated, then the signals received converted into digital information; and predetermined delay laws may be applied to the digital information. The electronic circuits for operating the frequency change represent a considerable part of the price of the equipment.
Furthermore, a type of ring transducer probe is known in which the beam is generated by a group of transducer elements in the form of concentric rings. This arrangement has the advantage of a Bessel function "antenna diagram" (18 dB attenuation of the secondary lobes with respect to the main lobe). Proposals have even been made for reconstituting such rings from a flat transducer element array, so as to cause movements of these rings providing an ultrasonic mission sweep in a predetermined direction. This has the drawback of creating expensive and cumbersome probes, (like the linear arrays). Furthermore, the coupling is mediocre.